摘要：Most large earthquakes occur along an oceanic trench, where anoceanic plate subducts beneath a continental plate. Massive earthquakes with a moment magnitude, Mw, of nine have been known to occur in only a few areas, including Chile, Alaska, Kamchatka and Sumatra. No historical records exist of a Mw=9 earthquake along the Japan trench, where the Pacific plate subducts beneath the Okhotsk plate, with the possible exception of the AD 869 Jogan earthquake[1], the magnitude of which has not been well constrained. However, the strain accumulation rate estimated there from recent geodetic observations is much higher than the average strain rate released in previous interplate earthquakes[2-6]. This finding raises the question of how such areas release the accumulated strain. A megathrust earthquake with Mw=9.0 (hereafter referred to as the Tohoku-Oki earthquake) occurred on 11 March 2011, rupturing the plate boundary off the Pacific coast of northeastern Japan. Here we report the distributions of the coseismic slip and postseismic slip as determined from ground displacement detected using a network based on the Global Positioning System. The coseismic slip area extends approximately 400 km along the Japan trench, matching the area of the pre-seismic locked zone 4. The afterslip has begun to overlap the coseismic slip area and extends into the surrounding region. In particular, the afterslip area reached a depth of approximately 100 km, with Mw=8.3, on 25 March 2011. Because the Tohoku-Oki earthquake released the strain accumulated for several hundred years, the paradox of the strain budget imbalance may be partly resolved. This earthquake reminds us of the potential for Mw≈9 earthquakes to occur alongother trench systems, even if no past evidence of such events exists.Therefore, it is imperative that strain accumulation be monitored using a space geodetic technique to assess earthquake potential.%大部分强震都发生在海沟,那里是海洋板块向大陆板块俯冲的地方.大量矩震级MW9.0以上的地震发生在若干区域,包括智利,阿拉斯加,堪察加半岛和苏门答腊岛等.位于太平洋板块俯冲鄂霍茨克板块的日本海沟,历史记载上没有发生过MW9.0地震,除了至今震级还有争议的公元869年Jogan大地震[1](可能超过MW9.0).然而,根据最新的大地测量资料估算的应变积累速率比已发生的板块间地震的平均释放速率要高[2-6].这一发现又引发了这样的疑问:这些区域如何释放积累的应变.2011年3月11日发生的逆冲型MW9.0强震(下文中均称为日本东北大地震),撕裂了H本东北部的太平洋海岸板块边界.我们报道根据GPS网络监测到的地表位移确定的同震与震后滑动特征.同震滑移区域沿H本海沟方向长约400 km,与震前闭锁区域基本吻合[4].震后滑动不仅覆盖了同震滑移区域,而且扩展到了其邻区,甚至其影响已经达到100 km深度区域,引起2011年3月25 H的MW8.3地震.由于东北地震释放了积累数百年的应变,之前提到的应变积累与释放不平衡的问题可能得到部分解释.这次东北地震同时也提醒我们,即使在过去没有发生过强震的其它海沟地区,也有发生MW9.0强震的潜在危险.因此,利用空间对地形变观测技术监测应变积累,以确定强震潜在危险区是非常重要的.
